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Oil tank cleaning

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Oil Tank Cleaning


Environmental legislation is becoming increasingly restrictive over waste disposal. This requirement becomes even more important in the cleaning of tanks, as the removal of tank sludge is an expensive and time consuming step before achieving gas free certification.In most cases sludge in the bottom of the tanks is removed manually to carry out maintenance. ITW has patented a novel technology for asphaltene stabilization. Such technology makes use of chemical additives to be added to hydrocarbons and has proved very effective in many industrial applications.The technology has been suitably modified for tank cleaning and successfully tested in many cases of aboveground storage tank cleaning (e.g. fuel oil and crude oil tanks), as well as cargo tanks. Asphaltene stabilization achieves an improvement in sludge reuse, in that asphaltene association is reduced, compatibility with the receiving hydrocarbon is enhanced, precipitation does not occur and cracking of asphaltene is facilitated.  Manual removal of the sludge is a very expensive and time consuming operation, complicated by the fact that tanks are difficult to operate in. Manual cleaning always implies a special attention to safety of operations.According to ITW approach, sludge is removed by the addition of a chemical additive, which contains asphaltene stabilizers, patented by ITW. The formulation also includes paraffin solvents and fluidizing agents.The additive is utilized to help sludge penetration, thus favouring its solubilization into a carrier.  Sludge dissolution occurs due to the chemical action of the additive during recycling of the oil phase.After a brief description of the existing tank cleaning technologies, we will report some results achieved in the application of the novel technology. 

Tank cleaning existing technologies

Manual cleaning is the most widespread today’s method for cleaning tanks.    This method has many disadvantages in that it:  

is unsafe      generates a huge amount of wastes  is time-consuming    is costly  


Other methods have been developed to improve manual tank cleaning and, among the others, the most interesting of them are:

   Crude Oil Washing (COW) catalyst    Chemical cleaning    Robot machines


Although the above methods improve manual cleaning they do still have pitfalls.   

COW simply moves the sludge from one tank to another (it is a mechanical dispersion method). In some cases, reprocessed crude oil sludge led to an unscheduled shutdown of the topping unit of a refinery.   

For chemical cleaning, chemicals used until now are basically dispersants: again, they transfer the problem from one point to another.

Robot machines improve the safety and sometime the time of the operations, but they do not have any impact on sludge reduction, therefore generating the same amount of sludge.    



ITW uses patented asphaltene stabilizers to make sludge a reusable product.    The used chemical additive by itself is capable of stabilizing and solubilizing the sludge. The chemical is not a dispersant, so it doesn’t create any problem in downstream equipments.   

Moreover, ITW stabilizers/solubilizers:   

  do not contain any metallic compound    do not contain any catalyst poison for petroleum processes    do not contain any halogen compound    do not contain any carcinogenic compound    do not contain any compound which, at operating dosages, can be a poison for waste water treatment plants   do not contain any compound which can be harmful to plant metallurgies   

Therefore, the core of ITW processes are highly effective chemicals which are able to solubilize per se the sludge. This means, the sludge will be solubilized chemically, i.e. stabilized permanently, with no danger of subsequent precipitation. To improve the performance of the chemical (especially in large tanks), a modified Crude Oil Washing is also used together with chemical stabilization of the sludge. 

Case history 1 A 5.000 m3 fuel oil tank needed to be cleaned after almost 20 years operations.    The tank was emptied with an external pump up to 50 cm, hence under the suction limit of tank pump. A certain amount of fuel oil was left above the sludge. Before starting the addition of the chemical, fuel oil was analyzed for SHF (Sediments by Hot Filtration - IP 375), which resulted as Not Filterable (i.e. no oil filtered through a Whatman GF/A filter of nominal porosity of 2µm).    This accounted for the asphaltene being a precipitate, hence strongly associated. Moreover, sludge contained a huge amount of catalyst fines (deriving from the blending during fuel oil formulation with decant oil from Fluid Catalytic Cracking Unit). Therefore a strong aggregation between sediments (substantially cat fines) and asphaltene occurred.    In such a case, precipitated asphaltene incorporated cat fines creating not filterable macrostructures. This phenomenon was operatively well known, as during fuel oil combustion frequent problems of burners and filters plugging occurred. Once suction of pumpables was ended (therefore leaving about 80 m3 of sludge), the chemical was added directly in the tank, by connecting the suction of the recycle pump with the container of the chemical. No carrier was utilized as diluent, so the chemical was injected directly in the not filterable fuel oil above the sludge.    Tank recycle started, and during recycle the quality of the oil phase (fuel oil + sludge) was analyzed. After one day recycle, SHF of the sludge was 3 %. Such result is extremely important, as it highlights the reactivity of the chemical: the fact that the oil phase went from not filterable to filterable is a clear demonstration of the stabilizing effect of the chemical.    The chemical has stabilized asphaltene to such an extent that they were not more aggregated, hence filterable; in such a way the sludge was freed from cat fines, which contributed to the formation of not filterable macrostructures. Release of sediments from asphaltene could be not possible, when asphaltene structure would have not been disjointed. The concept can be better illustrated as follows:   


By continuing recycling, sediments in the sludge increased with time during recycle: 7% the second day and 18 % the sixth day. The chemical effect is then evident: the additive has stabilized asphaltene in the sludge, thus allowing release of catalyst fines which were bound to them. It is important to note that the oil phase achieved from the sludge after ITW treatment was filterable, and SHF consisted almost entirely of cat fines. The oil phase of the sludge after ITW treatment has always been filterable and sediments increased gradually, once asphaltene were attacked from the chemical. In practical terms, the word filterable means, the oil did not contain sediments having dimension greater than 2 µm (nominal porosity of the filter utilized to measure SHF). At the end of recirculation, additivized and stabilized sludge was transferred into another fuel oil tank. The transfer has been performed gradually, by completing the operation in about 6 hours (rate of about 15 T/h). This in order to reduce potential operating problems, in that the sludge was transferred directly in the bottom of the receiving tank, and hence fed the boiler immediately (the charge pump sucked from the bottom of the tank). The sludge was entirely transferred into the receiving tank, and the level of the cleaned tank deepened down to zero.This was also confirmed by visual inspection from manways. The chemical additive has then solubilized and stabilized all the sludge (80 m3) present in the tank after only 6 days recycle. To confirm the success of the treatment, the sludge fed in the boiler gave no rise to any operating problem, in that free cat fines were easily stopped by hot filters, which did not suffer any fouling problem. The filters were able to dispose of stopped sediments in their normal cleaning time (20 min).    Pre-heaters did not suffer any fouling problem too.    Still more noticeable, is that no combustion and/or no burners plugging problems arose, which is a great success even compared to the normal combustion problems encountered during fuel oil combustion (burners, filters and pre-heaters plugging).    Any of the abovementioned operating problems would have been, in a way, justifiable as the boiler was fed with tank sludge, but asphaltene stabilization achieved with the chemical has been so effective to give rise to no pitfall in the combustion of the sludge. To further confirm the above, burners pressure had no increase during sludge combustion: on the contrary, it decreased due to the stabilizing action of the chemical.    Flame characteristics have been always at best during combustion of additivized sludge: flame has been clear and flame pattern has been regular, without sputtering.    Particulate matter emissions during combustion of additivized sludge have not increased; on the contrary they have slightly decreased, once blowing and load change are excluded. NOx emissions have been significantly decreased during combustion of additivized sludge.   

Case history 2 A power station had the need of cleaning the service tank, as it had not been cleaned since boiler construction (roughly 30 years).    Manual cleaning was not the solution however, as boiler turnaround was scheduled for only 20 days and an important revamping had to be implemented. As manual tank cleaning is a dirty, time consuming and almost unsafe operation, the management decided to test ITW technology. The purpose of the test was to have indications of cleaning during additivation of an ITW chemical. To give more added value to the application, ITW formulated a tailor made chemical, containing both asphaltene stabilizers and combustion catalysts. ITW fuel oil stabilizer and catalyst (hereinafter referred to as “ITW additive”), was injected upstream the service storage tank; the additivized fuel oil entered the tank from the bottom.  After about one and a half month of treatment a digging was performed in the tank with the following results: a layer of 20 cm solids, 30 cm of sludge with not measurable viscosity at 100 °C and 150 cm of fuel oil more viscous than the one above.  These results were interpreted in the sense that ITW additive, by entering the tank from the bottom, was solubilizing the sludge, so this solubilized sludge rendered the lower portion of the oil more viscous. After still another one month of treatment a further digging was performed in the tank and the results were surprising: the solid layer had disappeared, the same for the very viscous sludge. At their place a single not viscous sludge layer (viscosity 132 cSt at 100 °C) was found. The amazing results in tank cleaning were also confirmed by those in preflame and postflame zones of the boiler. As ITW additive contains both asphaltene stabilizers and combustion catalysts it performs its action also downstream the tank.  Actually both preflame and postflame zones were cleaner than in the previous situation.    In particular, hot filters ∆P was nearly nil after cleaning; on a normalized basis, the filter didn’t increase its ∆P.  Fouling factor monitoring in the pr-eaters revealed no increase in fouling. The most significant improvements in boiler operating parameters can be summarized as follows:   combustion chamber pressure decreased from about 260 mmH2O to about 230 mmH2O     flue gas pressure drop in the Ljungstroem decreased from about 100 mmH2O to an average of   about 70 mmH2O    air temperature at Ljungstroem outlet increased from about 395 °C to about 403 °C     normalized burners pressure was almost constant. The improved cleanliness of the boiler translated into improved combustion efficiency: specific steam production for Kcal of incoming fuel improved from about 0.0566 tons steam/fuel Kcal to about 0.0574 tons steam/fuel Kcal with an improvement of about 1%.    Apart from being tremendously effective in improving both pre- and post-flame cleanliness, ITW additive is also effective in reducing flue gas emissions.   


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